This invention relates to thin film resistors. More specifically, this invention relates to thin film resistors which have a controlled TCR ranging from minus to positive degrees kelvin.
Useful electronic circuits contain a combination of both passive components, which only transmit energy, consuming a part of it and active components, such as transistors. It is important that the passive components are compatible with the active components both in fabrication and in circuit performance. This is especially important when preparing miniaturized circuits by thin-film techniques.
Resistors for use in such circuits must have a well characterized resistivity and temperature coefficient of resistance, hereinafter referred to as TCR. The TCR is a measure of the change in resistance with respect to the change in operating temperature of the resistor and is commonly expressed in units of ppm/K. A resistor with a positive, negative or zero TCR may be required depending on the type of circuit and its particular applications. Moreover, the bulk and sheet resistivities of these components should be as independent of the TCR as possible. Thus, it is preferable that methods of fabricating thin-film resistors in addition to the requirements mentioned above, be able to produce resistors of preselected resistivity and TCR.
Present processes for preparing thin-film resistors includes the plating of nickel-chromium alloys on a substrate by thermal-evaporation of nickel-chromium compounds in a vacuum, or by the ion-bombardment of a nickel-chromium target. However, if evaporation resistors are made from metals with resistivity on the order of 10.sup.-4 .OMEGA.cm, high resistance values can only be achieved by the formation of very thin films which are often discontinuous. Tantalum, titanium and niobum have also been used, being deposited by cathode sputtering so as to possess resistance lower than the one required. For high values of resistance, use is also made cermet films which are mixtures of metals and dielectrics. With cermet films, resistivity increases with the dielectric content and may vary over a wide range. These films are usually deposited on ground porcelain tubes or plates.
Of the most commonly used thin film resistor materials, the alloy Ni(80)Cu(20) has a resistivity of about 110 .mu..OMEGA.cm and a TCR of 85 while Ni(76), Cu(20), Al(2), Fe(2) has a resistivity of 133 .mu..OMEGA.cm and a TCR of about 5.
However, all of these thin film resistors are prepared from alloys and it is difficult to prepare alloys having a precise composition, so as to combine resistivity and TCR, by any of the thin film deposition techniques. Thus, the reproducibility and hence the ability to control the TCR and resistivity is difficult, if not impossible to achieve.
U.S. patent application Ser. No. 202,083 filed Oct. 30, 1980 and assigned to the common assignee, (incorporated herein by reference) described a new material which is a coherent multilayer crystal containing at least two elements which are metals, each layer consisting of a single crystalline element at least 2 .ANG. in thickness, the elements being materials which will grow epitaxially on each other without forming intermetallic compounds.